There are 20 percent oxygen in our atmosphere, what if the oxygen is higher above 20 percent but instead was 35 percent oxygen. If so would an advanced civilization exist in this environment? Would metal rust quicker?

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    $\begingroup$ I don't remember the exact numbers, but nasa says that something like 30 or 35% percent is the absolute highest safe amount for a space shuttle. Above that fires and explosions would sweep your planet until the levels were reduced. $\endgroup$
    – cybernard
    Commented Dec 29, 2016 at 2:24
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    $\begingroup$ O2 may have peaked at 0.35 in the Permo-Carboniferous rstb.royalsocietypublishing.org/content/361/1470/903.full.pdf $\endgroup$
    – James K
    Commented Dec 29, 2016 at 3:10

3 Answers 3


Oxygen level of 35% at 1 bar (35 kPa O2) would be dangerous. Specifically,

At levels below 15% wildfires could not have spread. However, at levels significantly above 25% even wet plants could have burned, while at levels around 30 to 35%, as have been proposed for the Late Paleozoic, wildfires would have been frequent and catastrophic."Science Daily

Oxygen toxicity is a borderline problem at 35 kPa. Although 50 kPa is considered safe Oxygen Toxicity 60kPa is considered harmfull. There are no large-scale studies of long-term exposure to this level of oxygen, so I would expect to see at problems in at least some of the susceptible populations (sick, elderly, etc.) due to additional free radical damage, etc.

Yes, things would rust faster. As a first order approximation, reaction speeds tend to be linear against the concentration of the scarce reactant in a reaction. Concentration and Chemical Reaction Rate This rule is often broken in specific situations. Consider the classic test for O2, does it re-ignite a smoldering ember? Clearly a far faster reaction than the ratio of O2 pressure alone would suggest. Why? Because the reaction is strongly exothermic and the increased temperature also drives a much faster reaction rate.

At the level of simple rusting, I would not expect a huge deviation from the linear model, however in such types of chemistry there are always other things to consider such as surface area, mixing rates, heat loss, etc.

If you allow for redesign, trees could be less flammable, people could have better mechanism for free radical damage, etc., so it is up to the worldbuilder to determine how much of a problem exists.

Now as far as technology is concerned the ambient O2 would make fire more easily started, sustained, and hotter. This would be a large plus for early metallurgy as iron and steel working would be considerably easier as simple furnaces would be all that is needed. By the time you reach modern technology, it is mostly a non-issue as it is common to control the working atmospheres as needed in industrial processes, it may be a little more expensive in some cases, but it would just be the way it is.


There are problems with flammability

Here is an oxygen safety guide from a compressed gas vendor.

Among the relevant statements:

If the surrounding air becomes oxygen-enriched, the same changes in fire chemistry take place. In other words, materials become easier to ignite because their autoignition temperatures begin to decrease and their flammability ranges increase. Remember that materials which do not ordinarily burn in air may ignite, and materials that do burn in air will burn hotter and faster.

Oxygen is an explosive hazard when at elevated levels. If you click on the pdf, you can see a cotton shirt explode in a fireball when a flame is touched to it in an oxygen-enriched environment. Obviously, this poses a significantly increased fire hazard for civilizations and cities.

Here is a 1996 NASA Safety Standard for handling oxygen. While it states

To date, no single test has been developed that can produce either absolute ignition limits of consistent relative ratings for all materials

there are several guidelines within. In concentrated aluminum and iron alloys have been ignited by particulate impact of aluminum particles, while nickel and copper alloys are not ignited. Aluminum, tin, lead, and titanium have been ignited experimentally by mechanical impact, and also by friction of improperly lubricated or galling mechanical parts. Electrical arcing can ignite insulation in high oxygen concentrations. Regarding metals, titanium, beryllium, magnesium, and mercury all cannot be used in high oxygen environments as they are likely to ignite.

Many things will be much more flammable in your high oxygen environment. In particular, the heavily utilized metals iron and aluminum (though not steel) have significantly increased chances to auto-ignite in high oxygen environments, which would impact the development of technology in unpredictable ways.

  • $\begingroup$ How would the technology of a high oxygen world would be like $\endgroup$
    – User2988
    Commented Dec 29, 2016 at 4:39
  • $\begingroup$ @User2988 That is too broad a topic for this site. If that is really what you want to know, then this question will probably get closed as 'too broad.' $\endgroup$
    – kingledion
    Commented Dec 29, 2016 at 4:49

Generally you would be ok, vegetation would be more plentiful and lifeforms would probably be stronger. Fires would still burn slightly faster, ideally, the industrial revolution would reduce the total oxygen by a couple more percent just to be on the safe side.

Having a high humidity would prevent fires from either starting and/or would die quickly.


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